<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Document</title>
  <style>
    /*完善下列代码*/
    .per{
     width: 300px;
     height: 300px;
     background: red;
     margin-left:200px;
     margin-top: 200px;
     position: relative;
   }

   .son{
     width: 100px;
     height: 100px;
     background: blue;
     position: absolute;
     left: 0;right: 0;
     top: 0;bottom: 0;
     margin: auto;
   }
  </style>
</head>
<body>
  <div class="per">
    <div class="son"></div>
  </div>
</body>
</html>

<!-- <!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Document</title>
  <style>
    *{
      padding: 0;
      margin: 0;
    }
    .test{
      width: 100px;
      height: 100px;
      background: red;
      /*此处写代码*/
      position: absolute;
      right: 200px;
      bottom: 100px;  
    }
  </style>
</head>
<body>
  <div class="test"></div>
</body>
</html> -->

<!-- <!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        .per{
            width: 300px;
            height: 300px;
            background-color: aquamarine;
            margin-left: 200px;
            margin-right: 200px;
        }
        .son{
            width: 100px;
            height: 100px;
            background-color:rebeccapurple;
            position: absolute;
            left: 0px;
            right: 0px;
        }
    </style>
</head>
<body>
    <div class="per">
        <div class="son"></div>
    </div>
</body>
</html> -->